Recently, energy-saving has been actively promoted for environmental measures. For portable equipment using a battery, such as a mobile phone, a digital camera, and so on, it is especially important to establish energy savings to have a longer battery life. Such portable equipment widely uses a step-down type switching regulator which includes an inductor because it has a high efficiency and a capability to make it compact.
The switching regulator can perform with a high efficiency when a load is relatively heavy. However, the switching regulator decreases in efficiency when the portable equipment is in a standby mode and in a condition when a load is relatively light, for example, in a sleep mode, because the switching regulator itself requires relatively large power consumption.
In background portable equipment, an exchanging mechanism is employed to exchange a control operation mode between a PWM (pulse width modulation) control operation mode and a PFM (pulse frequency modulation) control operation mode at the light load condition so that the power consumption is saved and the efficiency can be kept. However, the response speed to the change of the output voltage may be relatively slow using such background switching regulator. Further, a relatively large noise may be generated at the timing of every abrupt change of the load current.
To improve the switching response, a higher performance circuit working with a higher clock frequency is generally employed. However, the higher performance circuit causes a cost penalty because of a necessity of a complex circuit to drive high speed switches. Further, there may be an upper limit of the clock speed for the circuit.
Other background switching regulators include a series regulator at a last stage of the switching regulator circuit to reduce a deviation of the output voltage. There may also be a cost penalty due to a large circuit size for the installation of the series regulator circuit.